NMR logging of natural gas reservoirs

摘要

A lithology-independent method of well logging to estimate the pore size within a formation containing natural gas. The method makes use of a pulsed NMR logging tool utilizing a recovery time that is greater than the relaxation time of hydrocarbon gas within the formation. In a preferred embodiment, two different NMR logs are obtained: one with a recovery time significantly shorter than the longitudinal relaxation time, T1, of the gas and one with a recovery time equal to or longer than T1. The amplitudes of the two logs are subtracted to determine a restricted diffusion coefficient of the gas. The restricted diffusion coefficient can then be used to estimate pore size and permeability of the formation by means of a relationship between measured restricted diffusion coefficient, surface to volume ratio, and estimated bulk diffusion ratio of the gas.